Method of making a cylinder block with unlined piston bores

ABSTRACT

A cylinder block for an axial piston pump or motor is formed from a steel material. The block includes a plurality of bores that each have an internal bore surface of a predefined final material surface finish. The bores are subjected to various machining and heat treating processes to provide a bore surface formed from the same material as the cylinder block, and which has the desired final surface finish. Pistons are mounted within each of the bores for axial movement relative to the cylinder block. Each of the pistons is formed from a steel material and has an exterior surface that is in sliding contact with the internal bore surface. The subject cylinder block is processed to provide a steel-to-steel interface between the bore and piston and eliminates the need for liners to be specially formed or installed within each piston bore.

BACKGROUND OF THE INVENTION

This invention relates to a cylinder block for an axial piston pump ormotor that is subjected to a unique manufacturing process to providepiston bores that do not require liners.

Axial piston pumps and motors are used in various applications in theaerospace industry. These pumps and motors are subjected to severe loadrequirements and often operate in harsh environmental conditions. Eachpump and motor traditionally includes a cylinder block with a pluralityof piston bores formed therein that each receive a piston in areciprocating relationship. The block is coupled for rotation which ashaft and works in conjunction with the reciprocating pistons to operateas a pump or motor.

The loading and environmental conditions in which these pumps and motorsoperate exert high levels of stress on the cylinder block and pistons.High stress levels can result in increased piston bore wear and canultimately lead to cracking or fracture within the cylinder block. Toincrease reliability and reduce maintenance and service operations it isdesirable to have a minimal amount of wear within each piston bore.

Traditionally, the piston bores have inserts installed within each boreto provide an improved frictional surface for the piston. One example isshown in U.S. Pat. No. 3,319,575, which utilizes bushings in each boreto reduce wear. It is difficult to manufacture the bushings to the tighttolerances required for the application. It also increases assembly timeand cost to perform multiple insert operations on the cylinder block toinstall the bushings in the bores. Another example is shown in U.S. Pat.No. 6,180,183, which utilizes a process for casting bronze piston linersin each of the bores. This process is time consuming and expensive.

Thus, it is desirable to provide method for manufacturing a cylinderblock that eliminates the need for piston bore inserts or liners. It isalso desirable for the process to reduce costs and assembly time inaddition to overcoming the above referenced deficiencies.

SUMMARY OF THE INVENTION

The subject invention provides a cylinder block for an aircraft pump ormotor that includes unlined piston bores. The cylinder block is formedfrom a predetermined material, such as steel, and includes a pluralityof piston bores formed within the cylinder block. Each of the bores hasa bore engagement surface having a predefined material surface finish. Apiston is mounted within each one of the bores. Each piston defines anengagement piston surface that is in direct sliding contact with theengagement bore surface.

The method for making the cylinder block includes the following steps.The cylinder body is turned to a rough shape. Each of the piston boresare machined to an initial rough shape. The cylinder is heat treated toachieve a desired hardness. Then the bore is finish machined to apredetermined surface finish to define a final bore surface for directengagement with a piston surface.

Additional steps include attaching a valve plate to one end of thecylinder block. Preferably, the valve plate is diffusion bonded to thecylinder block by applying a predetermined pressure force against thevalve plate and heating the valve plate and cylinder block to apredetermined temperature to achieve a predefined bond strength.

The subject invention provides an improved cylinder block thateliminates the need for piston bore inserts or liners. These and otherfeatures of the present invention can be best understood from thefollowing specification and drawings, the following of which is a briefdescription.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic exploded view of a cylinder block, shaft andpiston assembly made according to the inventive process.

FIG. 2 is a side view of the cylinder block.

FIG. 3 is a cross-sectional view of the cylinder block of FIG. 1 afterbroaching.

FIG. 4 is an end view of the block of FIG. 3 with a timing slot machinedin the outer surface.

FIG. 5A is a cross-sectional view of the block of FIG. 4 with a pistonbore.

FIG. 5B is an end view of the block of FIG. 5A.

FIG. 6A is an exploded view of a fixture assembly for diffusion bondingof the valve plate to the cylinder body.

FIG. 6B is an assembled view of FIG. 6A.

FIG. 7 is a cross-sectional view of the block of 6A after diffusionbonding.

FIG. 8A is an end view of the block of FIG. 7 with kidneys milled intothe valve plate.

FIG. 8B is a cross-sectional view of FIG. 8A.

FIG. 9 is cross-sectional view, partially broken away, of the cylinderblock body with a groove.

FIG. 10A is an end view of the final cylinder block assembly.

FIG. 10B is a cross-sectional view of the block of FIG. 10A.

DETAILED DESCRIPTION OF AN EXEMPLARY EMBODIMENT

A schematic view of a cylinder block assembly is shown generally at 10in FIG. 1. The cylinder block assembly 10 includes a body 12 that has afirst end 14 and a second end 16 facing in an opposite direction for thefirst end 14. Body 12 is preferably cylindrically shaped but can beformed in other configurations. The body 12 preferably includes a centerspline portion 18 for mating engagement with a splined shaft 20 thatdefines an axis of rotation 22. While a spline configuration ispreferred, it should be understood that other similar matingconfigurations known in the art could also be used. When the shaft 20 isinstalled within the center spline portion 18, the body 12 can berotated for use in a conventional pump or motor application. Preferably,the subject cylinder block 10 is used in aircraft power systems.

The main body 12 includes a plurality of piston bores 24 that definebore axes 26, which extend parallel to the axis of rotation 22. Thebores 24 are preferably spaced in an annular pattern about the axis ofrotation 22. Preferably, the cylinder block 10 includes nine (9) bores,however, a greater number or reduced number of bores can also be formedwithin the body 12. Each bore 24 is machined to a final bore surfacefinish 28. This process will be discussed in greater detail below.

Pistons 30 are mounted within each bore 24 in a reciprocatingrelationship to drive the pump or motor. Each piston 30 defines a pistonengagement surface 32 that is in direct sliding contact with the finalbore surface finish 28. Preferably, both the body 12 and the piston 30are formed from steel such that the contact between the block 10 and thepistons 30 is a steel-to-steel interface. While steel is the preferredmaterial, other similar materials known in the art can also be used.

As shown in FIG. 2, the block body 12, formed from a desired material,is cut to the proper length and is turned to a rough shape. As discussedabove, the body 12 is preferably turned into a cylindrical shape. Next,the splined bore 18 is broached, see FIG. 3. Next, a timing slot 34 ismachined into the outer surface 36 of the body 12, shown in FIG. 4. Thetiming slot 34 is used as a reference to correctly locate each of thepiston bores 24.

As shown in FIGS. 5A and 5B, a plurality of piston bores 24 are formedinto a rough shape. Preferably, each of the bores 24 is milled anddrilled to an initial size and shape. Preferably, the bores 24 areformed in an annular pattern about the center splined bore 18 such thateach bore axis 26 is parallel to the axis of rotation 22.

The face at the first end 14 is then lapped to achieve a desiredflatness. This first end 14 is preferably the end opposite from thesplined bore 18 that mates with the shaft 20. The lapping process alsoachieves the desired overall length L for the body 12, shown in FIG. 5A.

Next, a valve plate 40 is attached to the end opposite from the splinedbore 18. Preferably, the valve plate 40 is diffusion bonded to the body12 by applying a predetermined pressure force against the valve plate 40and body 12 and heating the valve plate 40 and body 12 to apredetermined temperature to achieve a predefined bond strength. Thepressure and heat are applied for a predetermined length of timedetermined by the desired bond strength.

As shown in FIGS. 6A and 6B the valve plate 40 is placed against thefirst end 14 of the body 12. The valve plate 40 and body 12 arepositioned between a pair of fixtures 42. A bolt 44 or other similardevice is inserting through the valve plate 40, through the body 12, andthrough both fixtures 42. The bolt 44 has a head portion 46 that extendsto a threaded end 48. After the bolt 44 has been inserted through theplate 40, body 12, and fixtures 42 the head portion 46 rests against oneof the fixtures 42 and the threaded end 48 extends beyond the otherfixture 42. A nut 50 is installed on the threaded end 48 and istightened against the other of the fixtures 42 to achieve thepredetermined pressure for the diffusion bonding process. Preferably,washers 52 are placed between the head portion 46 and the fixture 42 andbetween the nut 50 and the fixture 42. Preferably, gaskets 54 are placedbetween the valve plate 40 and the fixture 32 and between the body 12and the other fixture 42.

Diffusion bonding then occurs in a vacuum furnace (not shown) at apredetermined temperature for a predetermined length of time to achievea desired bond strength. The pressure, temperature, and length of timevaries depending on the type of materials and the size and weight of thebody 12 and plate 40. The assembly is removed from the furnace and thebolt 44 is unfastened. Then the body 12 is tempered to a hardness ofapproximately 50-55 RC.

Once the plate 40 is attached to the body 12. The external surface 36 ofthe body 12 is turned to an intermediate shape, shown in FIG. 7. Theexternal surface 56 of the plate 40 is also turned during this processsuch that the body 12 and the plate 40 are of the same outer diameter.Additional machining steps include grinding the center splined bore 18to a desired final form, grinding the outer surface 36 to anintermediate shape, and grinding the splined end 16 to a desired length.

Next, openings 60, referred to as kidneys, are milled into the plate 40,see FIG. 8A. The kidneys 60 are formed as elongated oval openings andare aligned over the piston bores 24. The kidneys 60 are milled all theway through the plate 42 and through the end 14 of the cylinder blockbody 12 as shown in FIG. 8B. The other pump or motor portion is mountedto the valve plate 40 opposite from the cylinder block body 12. Thekidneys 60 help disperse the oil between the pump or motor halves.

Drainage slots 62 and a groove 64 are milled into the end 14 of thecylinder block body 12, see FIG. 9. The slots 62 are in communicationwith the groove 64. As the cylinder block 10 rotates, there is some oilleakage. The leakage flows into the groove 64, which drains into theslots 62.

Next, the bores 24 are ground to a rough size and the outer surface 36is turned to a final shape. In the turning process, the timing slot 34is removed. Finally, the bores 24 are honed to a finished size and finalsurface finish 28, see FIGS. 10A and 10B. The final surface finish 28 iswithin a roughness average (Ra) of 1-30, however, the range of 6-16 Rais preferred.

The subject invention provides a method for forming a cylinder blockthat does not require piston bore liners or inserts. The bore surface 28is machined to a final surface finish that directly engages the piston30 in sliding contact. The piston to bore interface operates efficientlyunder severe environmental conditions and under high loads.

The aforementioned description is exemplary rather that limiting. Manymodifications and variations of the present invention are possible inlight of the above teachings. The preferred embodiments of thisinvention have been disclosed. However, one of ordinary skill in the artwould recognize that certain modifications would come within the scopeof this invention. Hence, within the scope of the appended claims, theinvention may be practiced otherwise than as specifically described. Forthis reason the following claims should be studied to determine the truescope and content of this invention.

I claim:
 1. A method for producing a cylinder block with unlined pistonbores comprising the steps of: (a) machining at least one bore withinthe cylinder block to a rough shape; (b) heat treating the cylinderblock to achieve a desired bore surface hardness; and (c) finishmachining the bore to a predetermined surface finish to define a finalbore surface for direct engagement with a piston surface.
 2. The methodaccording to claim 1 wherein the cylinder block includes a main bodyextending along a longitudinal axis with a first end and a second endfacing opposite from the first end, the method further including thestep of attaching a valve plate to one of the first or second ends priorto step (b).
 3. The method according to claim 2 wherein the step ofattaching the valve plate to one of the ends further includes diffusionbonding the valve plate to one of the first or second ends by applying apredetermined pressure force against the valve plate and cylinder block,and heating the valve plate and cylinder block to a predeterminedtemperature to achieve a predefined bond strength between the valveplate and the cylinder block.
 4. The method according to claim 3including the step of applying the pressure and heat for a predeterminedlength of time.
 5. The method according to claim 3 including the stepsof placing the valve plate against one of the first or second ends ofthe cylinder block; positioning the valve plate and cylinder blockbetween a pair of fixtures; inserting a bolt with a head portionextending to a threaded end, through the valve plate, through thecylinder block, and through both fixtures such that the head portionengages one of the fixtures and the threaded end extends beyond theother fixture; installing a nut on the threaded end, and tightening thenut against the other of the fixtures to achieve the predeterminedpressure.
 6. The method according to claim 1 wherein step (b) includestempering the cylinder block to achieve the desired bore surfacehardness.
 7. The method according to claim 1 including the step ofgrinding the bore to a rough size subsequent to step (b).
 8. The methodaccording to claim 1 wherein step (c) includes honing the bore to afinal size.
 9. The method according to claim 1 including the step ofproviding the cylinder block from a steel material.
 10. A method forproducing a cylinder block with unlined piston bores comprising thesteps of: (a) machining a steel block to form a cylindrical body with afirst end face and a second end face facing opposite from the first endface; (b) machining a plurality of bores within the cylindrical body toan initial rough (c) diffusion bonding a valve plate to the first endface of the cylindrical body; and (d) finish machining the bores to apredetermined surface finish to define a final bore surface for directengagement with a piston surface.
 11. The method according to claim 10including the step of tempering the valve plate and cylindrical body toa predetermined hardness subsequent to step (c).
 12. The methodaccording to claim 11 wherein step (a) further includes turning thecylindrical body to a rough shape and broaching a spline for engagementwith a rotating shaft through the center of the cylindrical body todefine a longitudinal axis of rotation.
 13. The method according toclaim 12 wherein step (b) further includes machining the bores in anannular pattern about the longitudinal axis with each bore axis beingparallel to the longitudinal axis of rotation and machining the bores tohave an open end facing the second end face and an enclosed end facingthe first end face.
 14. The method according to claim 12 wherein step(a) further includes machining a timing slot along an outer surface ofthe cylindrical body and wherein step (b) further includes machiningeach bore relative to the timing slot to form the annular pattern. 15.The method according to claim 12 including the step of lapping the firstend face to a predetermined flatness prior to step (c).
 16. The methodaccording to claim 15 wherein step (c) further includes applyingpredetermined pressure force to the valve plate and cylindrical body andheating the valve plate and cylindrical body to a predeterminedtemperature for a predetermined length of time to achieve a predefinedbond strength between the valve plate and the cylindrical body.
 17. Themethod according to claim 16 including the steps of placing the valveplate against the first end face of the cylindrical body; positioningthe valve plate and cylinder block between a pair of fixtures; insertinga bolt through the valve plate, cylinder block, and both fixtures;threading a nut on a distal end of the bolt; and tightening the nut toachieve the predetermined pressure.
 18. The method according to claim 16including the step of milling a plurality of openings through the valveplate and into the bores, with one opening aligned with each bore, todefine a fluid communication path between the cylindrical body and amating component mounted to an opposite side of the valve plate from thecylindrical body.
 19. The method according to claim 16 including thestep of grinding the bores to a rough shape after step (c).
 20. Themethod according to claim 19 wherein step (d) further includes honingthe bores to a final size and final bore surface at the predeterminedsurface finish.
 21. The method according to claim 20 including the stepof lapping valve plate to a predetermined flatness after step (d).